In general, a selective catalytic reduction (SCR) system refers to a system which reduces nitrogen oxide by purifying exhaust gas produced in diesel engines, boilers, incinerators and the like.
The selective catalytic reduction system allows nitrogen oxide contained in exhaust gas to react with a reducing agent by allowing both the exhaust gas and the reducing agent to pass through a reactor in which the catalyst is installed, thereby reducing the nitrogen oxide to nitrogen and water vapor.
The selective catalytic reduction system uses and directly injects urea as a reducing agent for reducing nitrogen oxide, or uses and injects ammonia (NH3) created through hydrolysis of urea.
However, in a case in which urea is injected directly to exhaust gas at a temperature lower than 250° C., there is a problem when a nozzle is clogged or a flow of exhaust gas is hindered due to by-products, such as biuret, cyanuric acid, melamine, and ammeline, which are produced when the urea is decomposed.
In addition, to create ammonia through hydrolysis of urea, a temperature in a hydrolysis chamber needs to be increased to a temperature required for a hydrolysis reaction by using an electric heater or a burner, and as a result, there is a problem in that a large amount of energy is separately consumed for the hydrolysis.